[sergiomanzur]

Add ROM-free execution coverage for the shared-tail return problem encountered during ISSD Native bring-up. Current main already handles this using tail-entry markers in the runtime, so this PR preserves that implementation and adds a regression rather than applying the older downstream per-wrapper decrement.

The test compiles an actual emitted past-end tail statement together with the production context-adoption functions and ancestor/post-return resolvers. It exercises one to three tail wrappers, normal returns, pre-RTS and post-RTS ancestor skips, unchanged LLE unwind sentinels, and discarded/consumed context.

Validation:

• python -m pytest tests/v2/test_tail_unwind_execution.py -q: 1 passed (12 tail-chain scenarios plus context checks).
• Mutation check: replacing guest-depth accounting with raw host-frame depth fails with 18 fixture failures.
• Mutation check: inserting the downstream wrapper decrement fails with 15 fixture failures, including sentinel propagation.
• Independent review found no blocking issues.

The test executes production resolver bodies and emitted wrapper code; its frame push/prologue and ordinary caller continuation are modeled by the fixture. It does not establish cross-title gameplay coverage. No runtime behavior, game ROM, or generated commercial-game source changes.

[mstan]

Test-only addition. The fixture exercises emitted tail wrappers and production ancestor resolvers; it preserves the existing runtime implementation and tests sentinel propagation and context consumption.

Validation: five-head integration passed 826 Python tests (2 skipped) and 109 Rust tests. After excluding #198 for its separate title-save compatibility blocker, the four-head combination passed all 69 focused tests. Linux, Windows, macOS and Lua CI are green for this reviewed head. No new cross-title gameplay playtest is claimed.